Secretary of Defense James Mattis has warned North Korea of an overwhelming response if it chooses to use nuclear weapons.

(SEOUL, SOUTH KOREA) U.S. President Donald Trump’s defense secretary warned North Korea on Friday of an “effective and overwhelming” response if it chose to use nuclear weapons, as he reassured South Korea of steadfast U.S. support.

“Any attack on the United States, or our allies, will be defeated, and any use of nuclear weapons would be met with a response that would be effective and overwhelming,” Defense Secretary James Mattis said at South Korea’s defense ministry, at the end of a two-day visit.

Mattis’ remarks come amid concern that North Korea could be readying to test a new ballistic missile, in what could be an early challenge for Trump’s administration.

North Korea, which regularly threatens to destroy South Korea and its main ally, the United States, conducted more than 20 missile tests last year, as well as two nuclear tests, in defiance of U.N. resolutions and sanctions.

The North also appears to have also restarted operation of a reactor at its main Yongbyon nuclear facility that produces plutonium that can be used for its nuclear weapons program, according to the U.S. think-tank 38 North.

“North Korea continues to launch missiles, develop its nuclear weapons program and engage in threatening rhetoric and behavior,” Mattis said.